It is known that metal oxides usually have hydroxyl groups on their surface. This is particularly the case if the oxides have been produced in an aqueous medium through precipitation of the metal ions as hydroxides or hydrated oxides, and subsequent dehydration. Depending on the respective metal, the metal oxides can have a basic, acidic or amphoteric character, i.e., the hydroxide groups can gain or lose protons. Measuring the pH of an aqueous suspension of the oxide determines the basic, acidic or amphoteric character of an oxide. The quantity of hydroxide groups on the surface can be determined through acid/base titration.
The hydroxide groups lend the oxides a more or less pronounced hydrophilic character. In the production of dispersions comprising fine-particle oxides, the hydroxide groups appear on the surface of a certain oxide particle in attractive interaction with hydroxide groups on the surface of adjacent particles, thus effecting a certain cohesion of the particles that leads to agglomerations and reduces the dispersibility of the oxide particles. It is known that this effect can be counteracted by the addition of cationic or anionic dispersion agents. Depending on the respective metal, these agents generate positive or negative charges on the particle surface that counteract the agglomeration in a desired manner, but, at least in very fine-particle dispersions, lead to spacing between particles that can approach or even exceed the order of magnitude of the particle diameter due to Coulomb repulsion. It is therefore difficult or impossible to produce the commonly-desired dispersions with very small particle diameters, and yet high solids contents.